Vapor phase growth method and vapor phase growth apparatus

ABSTRACT

A vapor phase growth method of embodiments includes: forming a first silicon carbide layer having a first doping concentration on a silicon carbide substrate at a first growth rate by supplying a first process gas under a first gas condition; forming a second silicon carbide layer having a second doping concentration at a second growth rate higher than the first growth rate by supplying a second process gas under a second gas condition; and forming a third silicon carbide layer having a third doping concentration lower than the first doping concentration and the second doping concentration at a third growth rate higher than the second growth rate by supplying a third process gas under a third gas condition.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Applications No. 2021-182164, filed on Nov. 8, 2021, theentire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

Embodiments relate to a vapor phase growth method and a vapor phasegrowth apparatus for supplying a gas to form a film.

BACKGROUND OF THE INVENTION

As a method of forming a high-quality semiconductor film, there is anepitaxial growth technique in which a single crystal film is grown on asubstrate, such as a wafer, by vapor phase growth. In a vapor phasegrowth apparatus using the epitaxial growth technique, a wafer is placedon a substrate holding portion in a reactor maintained at atmosphericpressure or lower pressure.

Then, while heating the wafer, a process gas such as a source gas, whichis a raw material for the semiconductor film, is supplied from the upperpart of the reactor to the wafer surface in the reactor, for example.Pyrolysis and chemical reaction of the source gas occur on the wafersurface, and an epitaxial single crystal film is formed on the wafersurface.

When forming a silicon carbide layer on the substrate by using theepitaxial growth technique, a crystal defect is generated in the siliconcarbide layer. If a crystal defect is present in the silicon carbidelayer, for example, the reliability of the semiconductor device formedon the silicon carbide layer is reduced, which causes a problem.Therefore, it is desired to reduce the crystal defect density in thesilicon carbide layer.

SUMMARY OF THE INVENTION

A vapor phase growth method of embodiments includes: forming a firstsilicon carbide layer having a first doping concentration on a siliconcarbide substrate at a first growth rate by supplying a first processgas containing a carrier gas into a reactor under a first gas condition;forming a second silicon carbide layer having a second dopingconcentration at a second growth rate higher than the first growth rateby supplying a second process gas containing a carrier gas into thereactor under a second gas condition after the forming the first siliconcarbide layer; and forming a third silicon carbide layer having a thirddoping concentration lower than the first doping concentration and thesecond doping concentration at a third growth rate higher than thesecond growth rate by supplying a third process gas containing a carriergas into the reactor under a third gas condition after the forming thesecond silicon carbide layer.

A vapor phase growth apparatus of embodiments includes: a reactor; acarrier gas supply pipe supplying a carrier gas to the reactor; a firstsource gas supply pipe supplying a first source gas containing silicon(Si) to the reactor; a second source gas supply pipe supplying a secondsource gas containing carbon (C) to the reactor; a first mass flowcontroller provided in the carrier gas supply pipe to control a flowrate of the carrier gas supplied to the reactor; a second mass flowcontroller provided in the first source gas supply pipe to control aflow rate of the first source gas supplied to the reactor; a third massflow controller provided in the second source gas supply pipe to controla flow rate of the second source gas supplied to the reactor; a pressureadjusting valve adjusting a pressure in the reactor; and a controlcircuit controlling the first mass flow controller, the second mass flowcontroller, the third mass flow controller, and the pressure adjustingvalve so that an average residence time of the carrier gas in thereactor is shorter than a transition time when switching the flow rateof at least one of the first source gas and the second source gassupplied to the reactor at the transition time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a vapor phase growth apparatus ofembodiments;

FIG. 2 is a schematic cross-sectional view of an example of a siliconcarbide layer formed by a vapor phase growth method of embodiments;

FIG. 3 is an explanatory diagram of the vapor phase growth method ofembodiments;

FIGS. 4A and 4B are explanatory diagrams of the vapor phase growthmethod of embodiments;

FIG. 5 is a schematic cross-sectional view of a silicon carbide layerformed by a vapor phase growth method of a first comparative example;

FIG. 6 is a schematic cross-sectional view of a silicon carbide layerformed by a vapor phase growth method of a second comparative example;

FIG. 7 is an explanatory diagram of the function and effect of the vaporphase growth method of embodiments;

FIG. 8 is a schematic diagram of a first modification example of thevapor phase growth apparatus of embodiments; and

FIG. 9 is a schematic diagram of a second modification example of thevapor phase growth apparatus of embodiments.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments will be described with reference to thediagrams.

In this specification, the same or similar members may be denoted by thesame reference numerals.

In this specification, the direction of gravity in a state in which avapor phase growth apparatus is installed so that a film can be formedis defined as “down”, and the opposite direction is defined as “up”.Therefore, “lower” means a position in the direction of gravity withrespect to the reference, and “downward” means the direction of gravitywith respect to the reference. Then, “upper” means a position in adirection opposite to the direction of gravity with respect to thereference, and “upward” means a direction opposite to the direction ofgravity with respect to the reference. In addition, the “verticaldirection” is the direction of gravity.

In addition, in this specification, “process gas” is a general term forgases used for forming a film on a substrate, and is a conceptincluding, for example, a carrier gas, a source gas, a dopant gas, anassist gas, and a mixed gas thereof. The carrier gas forms a mainstreamof gas after being introduced into the reactor, and gases such as asource gas, a dopant gas, and an assist gas may be used as gases to betransported onto a wafer in the reactor. In addition, the carrier gasmay be mixed with a gas, such as a source gas, a dopant gas, or anassist gas, by connecting a gas supply pipe before being introduced intothe reactor, thereby being also used as a gas to be transported to thereactor.

A vapor phase growth method of embodiments includes: forming a firstsilicon carbide layer having a first doping concentration on a siliconcarbide substrate at a first growth rate by supplying a first processgas containing a carrier gas into a reactor under a first gas condition;forming a second silicon carbide layer having a second dopingconcentration at a second growth rate higher than the first growth rateby supplying a second process gas containing a carrier gas into thereactor under a second gas condition; and forming a third siliconcarbide layer having a third doping concentration lower than the firstdoping concentration and the second doping concentration at a thirdgrowth rate higher than the second growth rate by supplying a thirdprocess gas containing a carrier gas into the reactor under a third gascondition.

In addition, a vapor phase growth apparatus of embodiments includes: areactor; a carrier gas supply pipe for supplying a carrier gas to thereactor; a first source gas supply pipe for supplying a first source gascontaining silicon (Si) to the reactor; a second source gas supply pipefor supplying a second source gas containing carbon (C) to the reactor;a first mass flow controller provided in the carrier gas supply pipe tocontrol a flow rate of the carrier gas supplied to the reactor; a secondmass flow controller provided in the first source gas supply pipe tocontrol a flow rate of the first source gas supplied to the reactor; athird mass flow controller provided in the second source gas supply pipeto control a flow rate of the second source gas supplied to the reactor;a pressure adjusting valve for adjusting a pressure in the reactor; anda control unit for controlling the first mass flow controller, thesecond mass flow controller, the third mass flow controller, and thepressure adjusting valve so that an average residence time of thecarrier gas in the reactor is shorter than a transition time whenswitching the flow rate of at least one of the first source gas and thesecond source gas supplied to the reactor at the transition time.

FIG. 1 is a schematic diagram of a vapor phase growth apparatus ofembodiments. A vapor phase growth apparatus 100 of embodiments is, forexample, an epitaxial growth apparatus for epitaxially growing a singlecrystal silicon carbide layer on a single crystal silicon carbidesubstrate.

The vapor phase growth apparatus 100 of embodiments includes a reactor10, a susceptor 12, a heater 14, a carrier gas supply pipe 15, a firstsource gas supply pipe 16, a second source gas supply pipe 17, a dopantgas supply pipe 18, an assist gas supply pipe 19, a first mass flowcontroller 25, a second mass flow controller 26, a third mass flowcontroller 27, a fourth mass flow controller 28, a fifth mass flowcontroller 29, an exhaust pipe 30, a pressure adjusting valve 31, anexhaust pump 32, and a control unit 34. The control unit 34 includes aprocess condition storage unit 34 a, an average residence timecalculation unit 34 b, a transition time determination unit 34 c, a flowrate instruction unit 34 d, and a valve opening instruction unit 34 e.

The reactor 10 is formed of, for example, stainless steel. The reactor10 has, for example, a cylindrical wall. A silicon carbide layer isformed on the surface of a wafer W in the reactor 10. The wafer W is anexample of a substrate.

The susceptor 12 is provided inside the reactor 10. The susceptor 12 hasa function of holding the wafer W. An opening may be provided at thecenter of the susceptor 12.

The heater 14 is provided inside the reactor 10. The heater 14 isprovided below the susceptor 12. The heater 14 has a function of heatingthe wafer W. In addition, a heater (not shown) arranged in parallel tothe side surface of the reactor 10 may be provided inside the reactor10. A heater (not shown) arranged in parallel to the side surface of thereactor 10 can heat the wafer through a hot wall (not shown) arrangedinside the heater.

The carrier gas supply pipe 15 is connected to the reactor 10. Aplurality of carrier gas supply pipes 15 may be connected to the reactor10. The carrier gas supply pipe 15 supplies a carrier gas to the reactor10. The carrier gas is, for example, a hydrogen gas, an argon gas, or ahelium gas. Only the carrier gas may be introduced into the reactor 10,or the carrier gas may be mixed with a gas, such as a source gas, adopant gas, or an assist gas, to be introduced into the reactor 10.

The first source gas supply pipe 16 is connected to the reactor 10. Thefirst source gas supply pipe 16 may be connected to the reactor 10through the carrier gas supply pipe 15 by being connected to the carriergas supply pipe 15. The first source gas supply pipe 16 supplies a firstsource gas containing silicon (Si) to the reactor 10. The first sourcegas is a raw material for forming a silicon carbide layer. The firstsource gas is, for example, a monosilane (SiH₄) gas.

The second source gas supply pipe 17 is connected to the reactor 10. Thesecond source gas supply pipe 17 may be connected to the reactor 10through the carrier gas supply pipe 15 by being connected to the carriergas supply pipe 15. The second source gas supply pipe 17 supplies asecond source gas containing carbon (C) to the reactor 10. The secondsource gas is a raw material for forming a silicon carbide layer. Thesecond source gas is, for example, a propane (C₃H₈) gas.

The dopant gas supply pipe 18 is connected to the reactor 10. The dopantgas supply pipe 18 may be connected to the reactor 10 through thecarrier gas supply pipe 15 by being connected to the carrier gas supplypipe 15. The dopant gas supply pipe 18 supplies a dopant gas to thereactor 10. The dopant gas contains an n-type impurity or a p-typeimpurity that serves as a dopant for the silicon carbide layer. Then-type impurity is, for example, nitrogen (N). The p-type impurity is,for example, aluminum (Al) or boron (B). The dopant gas of n-typeimpurities is, for example, a nitrogen gas. The dopant gas of p-typeimpurities is, for example, a trimethylaluminum ((CH₃)₃Al) gas or adiborane (B₂H₆) gas.

The assist gas supply pipe 19 is connected to the reactor 10. The assistgas supply pipe 19 may be connected to the reactor 10 through thecarrier gas supply pipe 15 by being connected to the carrier gas supplypipe 15. The assist gas supply pipe 19 supplies an assist gas to thereactor 10. The assist gas suppresses the clustering of silicon (Si),for example. The assist gas is, for example, a hydrogen chloride (HCl)gas.

The first mass flow controller 25 is provided in the carrier gas supplypipe 15. When a plurality of carrier gas supply pipes 15 are connectedto the reactor 10, the first mass flow controller 25 may be provided ineach carrier gas supply pipe 15. The first mass flow controller 25 has afunction of monitoring the flow rate of the carrier gas supplied to thereactor 10 and controlling the flow rate of the carrier gas. The carriergas is, for example, a hydrogen gas, an argon gas, or a helium gas.

The second mass flow controller 26 is provided in the first source gassupply pipe 16. The second mass flow controller 26 has a function ofmonitoring the flow rate of the first source gas supplied to the reactor10 and controlling the flow rate of the first source gas. The firstsource gas is, for example, a monosilane gas.

The third mass flow controller 27 is provided in the second source gassupply pipe 17. The third mass flow controller 27 has a function ofmonitoring the flow rate of the second source gas supplied to thereactor 10 and controlling the flow rate of the second source gas. Thesecond source gas is, for example, a propane gas.

The fourth mass flow controller 28 is provided in the dopant gas supplypipe 18. The fourth mass flow controller 28 has a function of monitoringthe flow rate of the dopant gas supplied to the reactor 10 andcontrolling the flow rate of the dopant gas. The dopant gas is, forexample, a nitrogen gas.

The fifth mass flow controller 29 is provided in the assist gas supplypipe 19. The fifth mass flow controller 29 has a function of monitoringthe flow rate of the assist gas supplied to the reactor 10 andcontrolling the flow rate of the assist gas. The assist gas is, forexample, a hydrogen chloride gas.

The exhaust pipe 30 is connected to the reactor 10. The exhaust pipe 30has a function of discharging the process gas from the reactor 10.

The pressure adjusting valve 31 has a function of adjusting the pressurein the reactor 10 to a desired pressure by controlling the flow rate ofthe process gas discharged from the reactor 10. The pressure adjustingvalve 31 is, for example, a throttle valve or a butterfly valve.

The exhaust pump 32 has a function of discharging the gas from thereactor 10. The exhaust pump 32 is, for example, a vacuum pump.

The control unit 34 controls the operation of the vapor phase growthapparatus 100. The control unit 34 controls, for example, the operationsof the first mass flow controller 25, the second mass flow controller26, the third mass flow controller 27, the fourth mass flow controller28, the fifth mass flow controller 29, the pressure adjusting valve 31,and the exhaust pump 32.

The control unit 34 is, for example, a control circuit. The control unit34 is, for example, an electronic circuit. The control unit 34 includes,for example, hardware and software.

The control unit 34 includes the process condition storage unit 34 a,the average residence time calculation unit 34 b, the transition timedetermination unit 34 c, the flow rate instruction unit 34 d, and thevalve opening instruction unit 34 e.

The process condition storage unit 34 a has, for example, a function ofstoring the process conditions when forming the silicon carbide layer onthe wafer W. The process conditions are, for example, the gasconditions, wafer temperature, reactor pressure, or duration of eachprocess step when forming the silicon carbide layer. The gas conditionsare, for example, a gas type or a gas flow rate. The process conditionstorage unit 34 a stores, for example, a reactor volume.

The process condition storage unit 34 a is, for example, a memorydevice. The process condition storage unit 34 a is, for example, asemiconductor memory.

The average residence time calculation unit 34 b has a function ofcalculating an average residence time tx in the reactor 10 of thecarrier gas. The average residence time tx of the carrier gas in thereactor 10 means an average time from when the carrier gas is suppliedinto the reactor 10 to when the carrier gas is discharged to the outsideof the reactor 10. The average residence time tx is expressed by thefollowing equation, for example. In addition, the carrier gas flow ratemay be approximately used as an exhaust flow rate, and the same appliesto the following description. In addition, the reactor volume in thefollowing equation is obtained by subtracting the volume occupied byvarious components arranged in the reactor from the volume of the spacesurrounded by the wall surface of the reactor 10, and the same appliesto the following description.

tx=reactor pressure [Pa]×reactor volume [m³]/carrier gas flow rate[Pa·m³/s]

For example, the average residence time calculation unit 34 b calculatesthe average residence time tx from the carrier gas flow rate, thereactor pressure, and the reactor volume stored in the process conditionstorage unit 34 a.

The average residence time calculation unit 34 b is, for example, anaverage residence time calculation circuit. The average residence timecalculation unit 34 b is, for example, an electronic circuit. Theaverage residence time calculation unit 34 b includes, for example,hardware and software.

The transition time determination unit 34 c has, for example, a functionof determining a transition time t required for transitioning from oneprocess step to the next process step when forming a silicon carbidelayer. The transition time determination unit 34 c has, for example, afunction of determining the length of the transition time t when theflow rate of the source gas supplied to the reactor 10 is switched atthe transition time t. The transition time t is, for example, a timerequired from the start of the change of the flow rate of the source gasto the end of the change.

For example, the transition time determination unit 34 c determines thetransition time t so that the average residence time tx in the reactor10 of the carrier gas is shorter than the transition time t. Thetransition time determination unit 34 c determines the transition time tbased on the average residence time tx of the carrier gas calculated bythe average residence time calculation unit 34 b.

The flow rate instruction unit 34 d has a function of giving aninstruction regarding the gas flow rate to the first mass flowcontroller 25, the second mass flow controller 26, the third mass flowcontroller 27, the fourth mass flow controller 28, and the fifth massflow controller 29, for example.

The flow rate instruction unit 34 d is, for example, a flow rateinstruction circuit. The flow rate instruction unit 34 d is, forexample, an electronic circuit. The flow rate instruction unit 34 dincludes, for example, hardware and software.

The valve opening instruction unit 34 e has, for example, a function ofgiving an instruction regarding a predetermined opening to the pressureadjusting valve 31 so that the reactor pressure is maintained at thepressure stored in the process condition storage unit 34 a.

The valve opening instruction unit 34 e is, for example, a valve openinginstruction circuit. The valve opening instruction unit 34 e is, forexample, an electronic circuit. The valve opening instruction unit 34 eincludes, for example, hardware and software.

The control unit 34 controls the first mass flow controller 25, thesecond mass flow controller 26, the third mass flow controller 27, andthe pressure adjusting valve 31, for example, so that the averageresidence time tx in the reactor 10 of the carrier gas expressed by thefollowing equation is shorter than the transition time t when switchingthe flow rate of at least one of the first source gas and the secondsource gas supplied to the reactor 10 at the transition time t.

tx=reactor pressure [Pa]×reactor volume [m³]/carrier gas flow rate[Pa·m³/s]

FIG. 2 is a schematic cross-sectional view of an example of a siliconcarbide layer formed by a vapor phase growth method of embodiments. Thesilicon carbide layer formed by the vapor phase growth method ofembodiments is formed on a silicon carbide substrate 50. The siliconcarbide substrate 50 is an example of a substrate.

The silicon carbide layer formed by the vapor phase growth method ofembodiments includes a first buffer layer 52, a first transition layer54, a second buffer layer 56, a second transition layer 58, and a driftlayer 60.

The silicon carbide substrate 50 (substrate), the first buffer layer 52(first silicon carbide layer), the first transition layer 54, the secondbuffer layer 56 (second silicon carbide layer), the second transitionlayer 58, and the drift layer 60 (third silicon carbide layer) aresingle crystal 4H-SiC silicon carbide. The silicon carbide substrate 50,the first buffer layer 52, the first transition layer 54, the secondbuffer layer 56, the second transition layer 58, and the drift layer 60contain nitrogen (N) that is an n-type impurity as a dopant.

The silicon carbide substrate 50 is an example of a substrate. The firstbuffer layer 52 is an example of the first silicon carbide layer. Thesecond buffer layer 56 is an example of the second silicon carbidelayer. The drift layer 60 is an example of the third silicon carbidelayer.

The nitrogen concentration of the silicon carbide substrate 50 is, forexample, equal to or more than 1E18/cm³ and equal to or less than2E19/cm³. The thickness of the silicon carbide substrate 50 is, forexample, equal to or more than 300 μm and equal to or less than 500 μm.

The first buffer layer 52 is provided on the silicon carbide substrate50. The first buffer layer 52 has, for example, a function of improvingthe crystallinity of the drift layer 60.

The nitrogen concentration of the first buffer layer 52 is, for example,equal to or more than 1E17/cm³ and equal to or less than 2E19/cm³. Thenitrogen concentration of the first buffer layer 52 is lower than, forexample, the nitrogen concentration of the silicon carbide substrate 50.The thickness of the first buffer layer 52 is, for example, equal to ormore than 0.05 μm and equal to or less than 0.50 μm.

The first transition layer 54 is provided on the first buffer layer 52.The first transition layer 54 is provided between the first buffer layer52 and the second buffer layer 56.

The nitrogen concentration of the first transition layer 54 is, forexample, equal to or more than 1E17/cm³ and equal to or less than2E19/cm³. The thickness of the first transition layer 54 is, forexample, equal to or more than 0.01 μm and equal to or less than 0.15μm.

The second buffer layer 56 is provided on the first transition layer 54.The second buffer layer 56 has, for example, a function of improving thecrystallinity of the drift layer 60.

The nitrogen concentration of the second buffer layer 56 is, forexample, equal to or more than 1E17/cm³ and equal to or less than2E19/cm³. The nitrogen concentration of the second buffer layer 56 islower than, for example, the nitrogen concentration of the siliconcarbide substrate 50. The thickness of the second buffer layer 56 is,for example, equal to or more than 0.5 μm and equal to or less than 5μm.

The second transition layer 58 is provided on the second buffer layer56. The second transition layer 58 is provided between the second bufferlayer 56 and the drift layer 60.

The nitrogen concentration of the second transition layer 58 is, forexample, equal to or more than 1E13/cm³ and equal to or less than2E19/cm³. The thickness of the second transition layer 58 is, forexample, equal to or more than 0.01 μm and equal to or less than 0.15μm.

The drift layer 60 is provided on the second transition layer 58. Asemiconductor device is formed in or on the drift layer 60.

The nitrogen concentration of the drift layer 60 is lower than thenitrogen concentration of the first buffer layer 52 and the nitrogenconcentration of the second buffer layer 56. The nitrogen concentrationof the drift layer 60 is, for example, equal to or more than 1E13/cm³and equal to or less than 2E16/cm³. The thickness of the drift layer 60is, for example, equal to or more than 5 μm and equal to or less than500 μm.

Next, an example of the vapor phase growth method of embodiments will bedescribed. In the vapor phase growth method of embodiments, the vaporphase growth apparatus 100 shown in FIG. 1 is used.

Hereinafter, a case where a single crystal 4H-SiC silicon carbide layerdoped with nitrogen as an n-type impurity is formed on the surface ofthe silicon carbide substrate 50 will be described as an example. Inaddition, hereinafter, a case where the carrier gas is a hydrogen gas,the first source gas is a monosilane gas, the second source gas is apropane gas, the dopant gas is a nitrogen gas, and the assist gas is ahydrogen chloride gas when forming the silicon carbide layer will bedescribed as an example.

FIGS. 3, 4A, and 4B are explanatory diagram of the vapor phase growthmethod of embodiments. FIG. 3 is a diagram showing the time change ofthe growth rate of the silicon carbide layer when the silicon carbidelayer is formed. FIG. 4A is a diagram showing the time change of the gasflow rate of the process gas when forming the silicon carbide layer.FIG. 4B is a diagram showing the time change of the carbon/silicon atomratio (C/Si) in the process gas when forming the silicon carbide layer.

The vapor phase growth method of embodiments includes a first processstep of forming the first buffer layer 52, a second process step offorming the first transition layer 54, a third process step of formingthe second buffer layer 56, a fourth process step of forming the secondtransition layer 58, and a fifth process step of forming the drift layer60. The first process step, the second process step, the third processstep, the fourth process step, and the fifth process step arecontinuously performed in the same reactor 10 without taking thesubstrate out of the reactor 10.

First, the susceptor 12 on which the wafer W is placed is loaded intothe reactor 10. The wafer W is the silicon carbide substrate 50. Thewafer W is an example of a substrate.

The nitrogen concentration of the silicon carbide substrate 50 is, forexample, equal to or more than 1E18/cm³ and equal to or less than2E19/cm³.

Then, the wafer W is heated by using the heater 14. The temperature ofthe wafer W is, for example, equal to or more than 1550° C. and equal toor less than 1750° C.

In the first process step, the first buffer layer 52 is formed on thewafer W. The first buffer layer 52 has a first nitrogen concentration.The first buffer layer 52 is formed by supplying a first process gascontaining a carrier gas into the reactor 10 under the first gascondition. The first buffer layer 52 is formed at the first growth rate.The first buffer layer 52 is formed between time t0 and time t1.

The first nitrogen concentration is, for example, equal to or more than1E17/cm³ and equal to or less than 2E19/cm³.

The first gas condition includes, for example, the type of process gassupplied into the reactor 10 and the flow rate of each process gas inthe first process step.

In the first process step, a hydrogen gas, a monosilane gas, a propanegas, a nitrogen gas, and a hydrogen chloride gas having a predeterminedflow rate are supplied into the reactor 10 as the first process gas.

The first growth rate is, for example, equal to or more than 1 μm/h andequal to or less than 10 μm/h. The first growth rate depends, forexample, on a silicon/hydrogen atom ratio (Si/H) in the process gas. Thefirst growth rate depends, for example, on the ratio between the flowrate of the monosilane gas and the flow rate of the hydrogen gas in theprocess gas. The silicon/hydrogen atom ratio (Si/H) in the first processstep is, for example, equal to or more than 5.28E−5 and equal to or lessthan 4.65E−4.

The carbon/silicon atom ratio (C/Si) under the first gas condition is,for example, equal to or more than 0.5 and equal to or less than 3.5.The chlorine/silicon atom ratio (Cl/Si) under the first gas conditionis, for example, equal to or more than 1 and equal to or less than 30.

The thickness of the first buffer layer 52 is, for example, equal to ormore than 0.05 μm and equal to or less than 0.50 μm.

In the second process step, the first transition layer 54 is formed onthe first buffer layer 52. The first transition layer 54 is formedbetween time t1 and time t2. The difference between the time t2 and thetime t1 is a first transition time ta.

The thickness of the first transition layer 54 is, for example, lessthan the thickness of the first buffer layer 52.

In the third process step, the second buffer layer 56 is formed on thefirst transition layer 54. The second buffer layer 56 has a secondnitrogen concentration. The second buffer layer 56 is formed bysupplying a second process gas containing a carrier gas into the reactor10 under the second gas condition. The second buffer layer 56 is formedat the second growth rate. The second buffer layer 56 is formed betweentime t2 and time t3.

The second nitrogen concentration is, for example, equal to or more than1E17/cm³ and equal to or less than 2E19/cm³.

The second gas condition includes, for example, the type of process gassupplied into the reactor 10 and the flow rate of each process gas inthe third process step.

In the third process step, a hydrogen gas, a monosilane gas, a propanegas, a nitrogen gas, and a hydrogen chloride gas having a predeterminedflow rate are supplied into the reactor 10 as process gases.

As shown in FIG. 3 , the second growth rate is larger than the firstgrowth rate. The second growth rate is, for example, equal to or morethan 40 μm/h and equal to or less than 100 μm/h. The second growth ratedepends, for example, on a silicon/hydrogen atom ratio (Si/H) in theprocess gas. The second growth rate depends, for example, on the ratiobetween the flow rate of the monosilane gas and the flow rate of thehydrogen gas in the process gas. The silicon/hydrogen atom ratio (Si/H)in the third process step is, for example, equal to or more than 5.28E−4and equal to or less than 4.65E−3.

As shown in FIG. 4A, the flow rate of the monosilane gas under thesecond gas condition is larger than the flow rate of the monosilane gasunder the first gas condition. In addition, the silicon/hydrogen atomratio (Si/H) in the second process gas under the second gas condition islarger than the silicon/hydrogen atom ratio (Si/H) in the first processgas under the first gas condition. Since the silicon/hydrogen atom ratio(Si/H) under the second gas condition is larger than thesilicon/hydrogen atom ratio (Si/H) under the first gas condition, thesecond growth rate tends to be larger than the first growth rate. Inaddition, the flow rate of the hydrogen gas under the second gascondition is, for example, equal to the flow rate of the hydrogen gasunder the first gas condition. In addition, the flow rate of thehydrogen gas under the second gas condition may be adjusted in the rangeof 0.90 times to 1.10 times the flow rate of the hydrogen gas under thefirst gas condition, for example.

As shown in FIG. 4A, the flow rate of the propane gas under the secondgas condition is larger than the flow rate of the propane gas under thefirst gas condition.

The carbon/silicon atom ratio (C/Si) in the second process gas under thesecond gas condition is, for example, equal to or more than 0.5 andequal to or less than 2.0. The chlorine/silicon atom ratio (Cl/Si) inthe second process gas under the second gas condition is, for example,equal to or more than 1 and equal to or less than 30.

As shown in FIG. 4B, for example, the carbon/silicon atom ratio (C/Si)in the second process gas under the second gas condition is smaller thanthe carbon/silicon atom ratio (C/Si) in the first process gas under thefirst gas condition.

The thickness of the second buffer layer 56 is, for example, equal to ormore than 0.5 μm and equal to or less than 5 μm.

In the second process step, the gas condition are switched from thefirst gas condition to the second gas condition. In the second processstep, the gas condition are switched from the first gas condition to thesecond gas condition during the first transition time ta. The firsttransition time ta is a time required from the start of the change ofthe first gas condition to the end of the change to the second gascondition.

As shown in FIG. 4A, in the second process step, for example, the flowrate of the monosilane gas changes in a direction of increasing. Inaddition, as shown in FIG. 4A, in the second process step, for example,the flow rate of the propane gas changes in a direction of increasing.In the second process step, for example, the flow rate of the hydrogengas is equal to the flow rate of the hydrogen gas under the first gascondition. In addition, for example, the flow rate of the hydrogen gasmay be adjusted in the range of 0.90 times to 1.10 times the flow rateof the hydrogen gas under the first gas condition.

In the second process step of forming the first transition layer 54, theaverage residence time tx in the reactor 10 of the hydrogen gas is setto be shorter than the first transition time ta. The average residencetime tx in the reactor 10 of the hydrogen gas is expressed by thefollowing equation.

tx=reactor pressure [Pa]×reactor volume [m³]/carrier gas flow rate[Pa·m³/s]

In the second process step, the average residence time tx in the reactor10 of the hydrogen gas is, for example, equal to or more than 0.5seconds and equal to or less than 10 seconds.

In the fourth process step, the second transition layer 58 is formed onthe second buffer layer 56. The second transition layer 58 is formedbetween time t3 and time t4. The difference between the time t3 and thetime t4 is a second transition time tb.

The thickness of the second transition layer 58 is, for example, lessthan the thickness of the second buffer layer 56.

In the fifth process step, the drift layer 60 is formed on the secondtransition layer 58. The drift layer 60 has a third nitrogenconcentration. The drift layer 60 is formed by supplying a third processgas containing a carrier gas into the reactor 10 under the third gascondition. The drift layer 60 is formed at a third growth rate. Thedrift layer 60 is formed between time t4 and time t5.

The third nitrogen concentration is lower than the first nitrogenconcentration and the second nitrogen concentration. The third nitrogenconcentration is, for example, equal to or more than 1E13/cm³ and equalto or less than 2E16/cm³.

The third gas condition are, for example, the type of process gassupplied into the reactor 10 and the flow rate of each process gas inthe fifth process step.

In the fifth process step, a hydrogen gas, a monosilane gas, a propanegas, a nitrogen gas, and a hydrogen chloride gas having a predeterminedflow rate are supplied into the reactor 10 as process gases.

As shown in FIG. 3 , the third growth rate is larger than the secondgrowth rate. The third growth rate is, for example, equal to or morethan 45 μm/h and equal to or less than 100 μm/h. The third growth ratedepends, for example, on a silicon/hydrogen atom ratio (Si/H) in theprocess gas. The third growth rate depends, for example, on the ratiobetween the flow rate of the monosilane gas and the flow rate of thehydrogen gas in the process gas. The silicon/hydrogen atom ratio (Si/H)in the fifth process step is, for example, equal to or more than 5.28E−4and equal to or less than 4.65E−3.

As shown in FIG. 4A, the flow rate of the monosilane gas under the thirdgas condition is preferably equal to the flow rate of the monosilane gasunder the second gas condition, but may be adjusted in the range of 0.95times to 1.40 times the flow rate of the monosilane gas under the secondgas condition. In addition, for example, the flow rate of the hydrogengas under the third gas condition is equal to the flow rate of thehydrogen gas under the second gas condition. In addition, for example,the flow rate of the hydrogen gas under the third gas condition may beadjusted in the range of 0.90 times to 1.10 times the flow rate of thehydrogen gas under the second gas condition.

As shown in FIG. 4A, the flow rate of the propane gas under the thirdgas condition is larger than the flow rate of the propane gas under thesecond gas condition.

The carbon/silicon atom ratio (C/Si) under the third gas condition is,for example, equal to or more than 0.5 and equal to or less than 2.0.The chlorine/silicon atom ratio (Cl/Si) under the third gas conditionis, for example, equal to or more than 1 and equal to or less than 30.

As shown in FIG. 4B, for example, the carbon/silicon atom ratio (C/Si)in the third process gas under the third gas condition is larger thanthe carbon/silicon atom ratio (C/Si) in the second process gas under thesecond gas condition. For example, by making the carbon/silicon atomratio (C/Si) under the third gas condition larger than thecarbon/silicon atom ratio (C/Si) under the second gas condition, thethird growth rate can be made larger than the second growth rate.

The thickness of the drift layer 60 is, for example, equal to or morethan 5 μm and equal to or less than 500 μm.

In the fourth process step, the gas condition are switched from thesecond gas condition to the third gas condition. In the fourth processstep, the gas condition are switched from the second gas condition tothe third gas condition during the second transition time tb. The secondtransition time tb is a time required from the start of the change ofthe second gas condition to the end of the change to the third gascondition.

As shown in FIG. 4A, in the fourth process step, for example, the flowrate of the propane gas changes in a direction of increasing.

In the fourth process step of forming the second transition layer 58,the average residence time tx in the reactor 10 of the hydrogen gas isset to be shorter than the second transition time tb. The averageresidence time tx in the reactor 10 of the hydrogen gas is expressed bythe following equation.

tx=reactor pressure [Pa]×reactor volume [m³]/carrier gas flow rate[Pa·m³/s]

In the fourth process step, the average residence time tx in the reactor10 of the hydrogen gas is, for example, equal to or more than 0.5seconds and equal to or less than 10 seconds.

The silicon carbide layer shown in FIG. 2 is formed by the vapor phasegrowth method described above.

Next, the function and effect of the vapor phase growth method and thevapor phase growth apparatus of embodiments will be described.

When a silicon carbide layer is formed on a substrate by using theepitaxial growth technique, a crystal defect is generated in the siliconcarbide layer. Examples of the crystal defect include basal planedislocation (BPD) and stacking fault (SF). If a crystal defect ispresent in the silicon carbide layer, for example, the reliability ofthe semiconductor device formed on the silicon carbide layer is reduced,which causes a problem. Therefore, it is desired to reduce the crystaldefect density in the silicon carbide layer.

FIG. 5 is a schematic cross-sectional view of a silicon carbide layerformed by a vapor phase growth method of a first comparative example.The vapor phase growth method of the first comparative example isdifferent from the vapor phase growth method of embodiments in that thesecond buffer layer 56 is not formed between the first buffer layer 52and the drift layer 60. That is, the vapor phase growth method of thefirst comparative example is different from the vapor phase growthmethod of embodiments in that the second buffer layer 56 having a highergrowth rate than the first buffer layer 52 is not formed between thefirst buffer layer 52 and the drift layer 60. In addition, in FIG. 5 , adefect present in the silicon carbide substrate 50 is not shown.

A transition layer 59 is formed between the first buffer layer 52 andthe drift layer 60.

As shown in FIG. 5 , according to the vapor phase growth method of thefirst comparative example, for example, it is easy to suppress theformation of a stacking fault SF from the interface between the siliconcarbide substrate 50 and the first buffer layer 52. Therefore, it ispossible to suppress the formation of the stacking fault SF extending tothe drift layer 60. The stacking fault SF in the drift layer 60 grows,for example, during the bipolar operation of the semiconductor device,and increases the electrical resistance of the drift layer 60.Therefore, the stacking fault SF reduces the reliability of thesemiconductor device, which causes a problem.

According to the vapor phase growth method of the first comparativeexample, it is possible to suppress the formation of the stacking faultSF extending to the drift layer 60. Therefore, it is possible tosuppress the reduction of the reliability of the semiconductor devicedue to the stacking fault SF formed by the epitaxial growth.

On the other hand, in the vapor phase growth method of the firstcomparative example, the basal plane dislocation BPD that reaches thesurface of the silicon carbide substrate 50 from the inside of thesilicon carbide substrate 50 is likely to be propagated to the firstbuffer layer 52 to reach the surface of the first buffer layer 52. Inaddition, the basal-plane dislocation BPD that reaches the surface ofthe first buffer layer 52 is likely to be propagated to the vicinity ofthe first buffer layer 52 in the drift layer 60. The basal planedislocation BPD propagating to the vicinity of the first buffer layer 52in the drift layer 60 is converted into a threading edge dislocation(TED) in the drift layer 60, for example.

As described above, the basal plane dislocation BPD is easily convertedinto the threading edge dislocation TED in the vicinity of the firstbuffer layer 52 in the drift layer 60. That is, the basal planedislocation BPD is present up to the vicinity of the first buffer layer52 in the drift layer 60. When the basal plane dislocation BPD ispresent in the drift layer 60, for example, the basal plane dislocationBPD is converted into the stacking fault SF during the bipolar operationof the semiconductor device. The stacking fault SF increases theelectrical resistance of the drift layer 60, for example. Therefore, thereliability of the semiconductor device is reduced, which causes aproblem.

As described above, according to the vapor phase growth method of thefirst comparative example, it is possible to reduce the density of thestacking fault SF in the drift layer 60 formed by the epitaxial growth.On the other hand, in the vapor phase growth method of the firstcomparative example, it is difficult to reduce the density of the basalplane dislocation BPD extending to the vicinity of the first bufferlayer 52 in the drift layer 60. Therefore, in the vapor phase growthmethod of the first comparative example, the reduction of thereliability of the semiconductor device due to the basal planedislocation BPD or the stacking fault SF formed by being converted fromthe basal plane dislocation BPD becomes a problem.

FIG. 6 is a schematic cross-sectional view of a silicon carbide layerformed by a vapor phase growth method of a second comparative example.The vapor phase growth method of the second comparative example isdifferent from the vapor phase growth method of embodiments in that thefirst buffer layer 52 is not formed between the silicon carbidesubstrate 50 and the second buffer layer 56. That is, the vapor phasegrowth method of the second comparative example is different from thevapor phase growth method of embodiments in that the first buffer layer52 having a lower growth rate than the second buffer layer 56 is notformed between the silicon carbide substrate 50 and the second bufferlayer 56. In addition, in FIG. 6 , a defect present in the siliconcarbide substrate 50 is not shown.

A transition layer 59 is formed between the second buffer layer 56 andthe drift layer 60.

As shown in FIG. 6 , according to the vapor phase growth method of thesecond comparative example, for example, the basal plane dislocation BPDextending from the interface between the silicon carbide substrate 50and the second buffer layer 56 is likely to be converted into thethreading edge dislocation TED in the second buffer layer 56. Therefore,it is possible to reduce the density of the basal plane dislocation BPDin the drift layer 60.

In addition, it is known that the influence of the threading edgedislocation TED on the characteristics of the semiconductor device isminor as compared with, for example, the basal plane dislocation BPD andthe stacking fault SF.

According to the vapor phase growth method of the second comparativeexample, it is possible to reduce the density of the basal planedislocation BPD in the drift layer 60. Therefore, it is possible tosuppress the reduction of the reliability of the semiconductor devicedue to the basal plane dislocation BPD.

On the other hand, as shown in FIG. 6 , in the vapor phase growth methodof the second comparative example, the stacking fault SF is likely to beformed from the interface between the silicon carbide substrate 50 andthe second buffer layer 56. For this reason, it is difficult to suppressthe formation of the stacking fault SF extending to the drift layer 60.Therefore, the reduction of the reliability of the semiconductor devicedue to the stacking fault SF becomes a problem.

As described above, according to the vapor phase growth method of thesecond comparative example, it is possible to reduce the density of thebasal plane dislocation BPD in the drift layer 60 formed by theepitaxial growth. Therefore, it is possible to suppress the reduction ofthe reliability of the characteristics of the semiconductor device dueto the basal plane dislocation BPD. On the other hand, in the vaporphase growth method of the second comparative example, it is difficultto reduce the density of the stacking fault SF extending to the driftlayer 60. Therefore, in the vapor phase growth method of the secondcomparative example, the reduction of the reliability of thesemiconductor device due to the stacking fault SF becomes a problem.

FIG. 7 is an explanatory diagram of the function and effect of the vaporphase growth method of embodiments. FIG. 7 is a schematiccross-sectional view of an example of the silicon carbide layer formedby the vapor phase growth method of embodiments.

In the vapor phase growth method of embodiments, the first buffer layer52 is formed at the first growth rate before the drift layer 60 isformed on the silicon carbide substrate 50, and then the second bufferlayer 56 is formed at the second growth rate higher than the firstgrowth rate. By forming the first buffer layer 52, it is easy tosuppress the formation of the stacking fault SF from the interfacebetween the silicon carbide substrate 50 and the first buffer layer 52.Therefore, it is possible to reduce the density of the stacking fault SFextending into the drift layer 60. In addition, by forming the secondbuffer layer 56, the basal plane dislocation BPD that reaches thesurface of the silicon carbide substrate 50 from the inside of thesilicon carbide substrate 50 is easily converted into the threading edgedislocation TED in the second buffer layer 56, so that it is possible toreduce the density of the basal plane dislocation BPD in the drift layer60. As a result, it is possible to reduce the densities of both thestacking fault SF and the basal plane dislocation BPD in the drift layer60. Therefore, according to the vapor phase growth method ofembodiments, it is possible to suppress the reduction of the reliabilityof the semiconductor device due to crystal defects.

The first growth rate when forming the first buffer layer 52 ispreferably equal to or more than 1 μm/h and equal to or less than 10μm/h, more preferably equal to or more than 2 μm/h and equal to or lessthan 8 μm/h, and even more preferably equal to or more than 3 μm/h andequal to or less than 6 μm/h. By making the first growth rate exceed thelower limit value described above, the time required for forming thefirst buffer layer 52 is shortened. As a result, the productivity of thesilicon carbide layer is improved. In addition, by making the firstgrowth rate fall below the upper limit value described above, thedensity of the stacking fault SF in the drift layer 60 can be furtherreduced.

From the viewpoint of setting the first growth rate when forming thefirst buffer layer 52 to be equal to or more than 1 μm/h and equal to orless than 10 μm/h, it is preferable that the silicon/hydrogen atom ratio(Si/H) when forming the first buffer layer 52 is equal to or more than5.28E−5 and equal to or less than 4.65E−4.

The second growth rate when forming the second buffer layer 56 ispreferably equal to or more than 40 μm/h and equal to or less than 100μm/h, more preferably equal to or more than 45 μm/h and equal to or lessthan 80 μm/h, and even more preferably equal to or more than 50 μm/h andequal to or less than 60 μm/h. By making the second growth rate exceedthe lower limit value described above, the density of the basal planedislocation BPD in the drift layer 60 can be further reduced. Inaddition, by making the second growth rate fall below the upper limitvalue described above, the crystallinity or surface morphology of thedrift layer 60 is improved.

From the viewpoint of setting the second growth rate when forming thesecond buffer layer 56 to be equal to or more than 40 μm/h and equal toor less than 100 μm/h, it is preferable that the silicon/hydrogen atomratio (Si/H) when forming the second buffer layer 56 is equal to or morethan 5.28E−4 and equal to or less than 4.65E−3.

The thickness of the first buffer layer 52 is preferably equal to ormore than 0.05 μm and equal to or less than 0.5 μm. By making thethickness of the first buffer layer 52 exceed the lower limit valuedescribed above, the density of the stacking fault SF in the drift layer60 can be further reduced. In addition, by making the thickness of thefirst buffer layer 52 fall below the upper limit value described above,the time required for forming the first buffer layer 52 is shortened. Asa result, the productivity of the silicon carbide layer is improved.

The thickness of the second buffer layer 56 is preferably equal to ormore than 0.5 μm and equal to or less than 5 μm. By making the thicknessof the second buffer layer 56 exceed the lower limit value describedabove, the density of the basal plane dislocation BPD in the drift layer60 can be further reduced. In addition, by making the thickness of thesecond buffer layer 56 fall below the upper limit value described above,the time required for forming the second buffer layer 56 is shortened.As a result, the productivity of the silicon carbide layer is improved.

The thickness of the second buffer layer 56 is preferably larger thanthe thickness of the first buffer layer 52. In other words, thethickness of the first buffer layer 52 is preferably smaller than thethickness of the second buffer layer 56. By reducing the thickness ofthe first buffer layer 52 having a low growth rate, the time requiredfor forming the first buffer layer 52 is shortened. As a result, theproductivity of the silicon carbide layer is improved.

In the second process step of forming the first transition layer 54, theaverage residence time tx in the reactor 10 of the hydrogen gas ispreferably shorter than the first transition time ta. The averageresidence time tx in the reactor 10 of the hydrogen gas is expressed bythe following equation.

tx=reactor pressure [Pa]×reactor volume [m³]/carrier gas flow rate[Pa·m³/s]

By making the average residence time tx in the reactor 10 of thehydrogen gas shorter than the first transition time ta, it becomes easyto control the film thickness or the doping concentration of the firsttransition layer 54.

In the fourth process step of forming the second transition layer 58,the average residence time tx in the reactor 10 of the hydrogen gas ispreferably shorter than the second transition time tb. The averageresidence time tx in the reactor 10 of the hydrogen gas is expressed bythe following equation.

tx=reactor pressure [Pa]×reactor volume [m³]/carrier gas flow rate[Pa·m³/s]

By making the average residence time tx in the reactor 10 of thehydrogen gas shorter than the second transition time tb, it becomes easyto control the film thickness or the doping concentration of the secondtransition layer 58.

The average residence time tx in the reactor 10 of the hydrogen gas ispreferably equal to or more than 0.5 seconds and equal to or less than10 seconds from the viewpoint of suppressing the formation of stepbunching of the silicon carbide layer. In addition, the averageresidence time tx in the reactor 10 of the hydrogen gas is preferablyequal to or less than 10 seconds from the viewpoint of reducing thethickness of the first transition layer 54 and the thickness of thesecond transition layer 58.

The carbon/silicon atom ratio (C/Si) under the second gas condition whenforming the second buffer layer 56 is preferably smaller than thecarbon/silicon atom ratio (C/Si) under the first gas condition whenforming the first buffer layer 52. In other words, the carbon/siliconatom ratio (C/Si) under the first gas condition when forming the firstbuffer layer 52 is preferably larger than the carbon/silicon atom ratio(C/Si) under the second gas condition when forming the second bufferlayer 56.

When forming a nitrogen-doped silicon carbide layer, it is possible tosuppress the introduction of nitrogen into the silicon carbide layer byincreasing the carbon/silicon atom ratio (C/Si) in the process gas. Thisis because the entry of nitrogen atoms into the carbon sites of siliconcarbide is suppressed by increasing the carbon/silicon atom ratio (C/Si)in the process gas. Such a phenomenon occurs under the condition inwhich the carbon/silicon atom ratio (C/Si) in the process gas on thewafer surface exceeds 1.

The carbon/silicon atom ratio (C/Si) under the third gas condition whenforming the drift layer 60 is preferably larger than the carbon/siliconatom ratio (C/Si) under the second gas condition when forming the secondbuffer layer 56.

By increasing the carbon/silicon atom ratio (C/Si) under the third gascondition when forming the drift layer 60, it is possible to reduce thedensity of carbon vacancies in the drift layer 60. The carbon vacanciesserve as a minority carrier lifetime killer when operating a bipolarsemiconductor device, for example. By reducing the density of carbonvacancies in the drift layer 60, it is possible to improve thecharacteristics of the bipolar semiconductor device.

In the vapor phase growth method of embodiments, the process gas whenforming the first buffer layer 52, the first transition layer 54, thesecond buffer layer 56, the second transition layer 58, and the driftlayer 60 preferable contains a hydrogen chloride (HCl) gas as an assistgas. Since the process gas contains a hydrogen chloride (HCl) gas, it ispossible to suppress the generation of silicon droplets or thedeterioration of the surface morphology of the silicon carbide layer. Bysuppressing the generation of silicon droplets or the deterioration ofthe surface morphology of the silicon carbide layer, for example, it ispossible to reduce the crystal defect density in the silicon carbidelayer.

The chlorine/silicon atom ratio (Cl/Si) in the process gas when formingthe first buffer layer 52, the first transition layer 54, the secondbuffer layer 56, the second transition layer 58, and the drift layer 60is preferably equal to or more than 1 and equal to or less than 30. Whenthe chlorine/silicon atom ratio (Cl/Si) satisfies the above range, it ispossible to further suppress the generation of silicon droplets or thedeterioration of the surface morphology of the silicon carbide layer.

The vapor phase growth apparatus 100 of embodiments includes the controlunit 34 that controls the first mass flow controller 25, the second massflow controller 26, the third mass flow controller 27, and the pressureadjusting valve 31 so that the average residence time tx in the reactor10 of the carrier gas expressed by the following equation is shorterthan the transition time t when switching the flow rate of at least oneof the first source gas and the second source gas supplied to thereactor 10 at the transition time t.

tx=reactor pressure [Pa]×reactor volume [m³]/carrier gas flow rate[Pa·m³/s]

Since the vapor phase growth apparatus 100 of embodiments includes thecontrol unit 34, it is easy to control the film thickness or the dopingconcentration of the transition layer.

As described above, according to the vapor phase growth method ofembodiments, it is possible to reduce the crystal defect density in thesilicon carbide layer. In addition, according to the vapor phase growthapparatus of embodiments, it is possible to improve the uniformity ofthe silicon carbide layer.

FIG. 8 is a schematic diagram of a first modification example of thevapor phase growth apparatus of embodiments. A vapor phase growthapparatus 110 of the first modification example shows anotherconfiguration example of the vapor phase growth apparatus 100 ofembodiments.

The vapor phase growth apparatus 110 includes a reactor 10, a susceptor12, a heater 14, a carrier gas supply pipe 15, a first source gas supplypipe 16, a second source gas supply pipe 17, a dopant gas supply pipe18, an assist gas supply pipe 19, a first mass flow controller 25, asecond mass flow controller 26, a third mass flow controller 27, afourth mass flow controller 28, a fifth mass flow controller 29, anexhaust pipe 30, a pressure adjusting valve 31, an exhaust pump 32, anda control unit 34.

In the vapor phase growth apparatus 110 shown in FIG. 8 , the firstsource gas supply pipe 16, the second source gas supply pipe 17, thedopant gas supply pipe 18, and the assist gas supply pipe 19 areconnected to the carrier gas supply pipe 15. Thus, the carrier gas maybe mixed with a gas, such as a source gas, a dopant gas, or an assistgas, to be introduced into the reactor 10.

In addition, in the vapor phase growth apparatus 110 shown in FIG. 8 ,the first source gas supply pipe 16, the second source gas supply pipe17, the dopant gas supply pipe 18, and the assist gas supply pipe 19 maybe connected to the reactor 10 through the carrier gas supply pipe 15.As described above, the first source gas supply pipe 16, the secondsource gas supply pipe 17, the dopant gas supply pipe 18, and the assistgas supply pipe 19 may be connected to the reactor 10 through thecarrier gas supply pipe 15 by being connected to the carrier gas supplypipe 15.

FIG. 9 is a schematic diagram of a second modification example of thevapor phase growth apparatus of embodiments. A vapor phase growthapparatus 120 of the second modification example shows still anotherconfiguration example of the vapor phase growth apparatus 100 ofembodiments.

The vapor phase growth apparatus 120 includes a reactor 10, a susceptor12, a heater 14, a carrier gas supply pipe 15, a first source gas supplypipe 16, a second source gas supply pipe 17, a dopant gas supply pipe18, an assist gas supply pipe 19, a first mass flow controller 25, asecond mass flow controller 26, a third mass flow controller 27, afourth mass flow controller 28, a fifth mass flow controller 29, anexhaust pipe 30, a pressure adjusting valve 31, an exhaust pump 32, anda control unit 34.

In the vapor phase growth apparatus 120 shown in FIG. 9 , a plurality ofcarrier gas supply pipes 15 are provided. Thus, a plurality of carriergas supply pipes 15 may be connected to the reactor 10.

In the vapor phase growth apparatus 120 shown in FIG. 9 , the firstsource gas supply pipe 16 and the assist gas supply pipe 19 areconnected to one of the plurality of carrier gas supply pipes 15. Thesecond source gas supply pipe 17 and the dopant gas supply pipe 18 areconnected to another one of the plurality of carrier gas supply pipes15. Still another one of the plurality of carrier gas supply pipes 15 isconnected to the reactor 10 without being connected to the first sourcegas supply pipe 16, the second source gas supply pipe 17, the dopant gassupply pipe 18, and the assist gas supply pipe 19.

As described above, only the carrier gas may be introduced into thereactor 10, or the carrier gas may be mixed with a gas, such as a sourcegas, a dopant gas, or an assist gas, to be introduced into the reactor10. In the vapor phase growth apparatus 120 shown in FIG. 9 , each ofthe plurality of carrier gas supply pipes 15 includes the first massflow controller 25. Thus, when a plurality of carrier gas supply pipes15 are connected to the reactor 10, the first mass flow controller 25may be provided in each carrier gas supply pipe 15.

In addition, since the vapor phase growth apparatus 100 of embodimentsand the vapor phase growth apparatuses of the modification examplesthereof include the control unit 34, it is possible to improve theuniformity of the transition layer and the silicon carbide layer closeto the transition layer. In the control unit 34, since the averageresidence time tx can be easily calculated and the transition time t canbe easily determined from the calculated average residence time tx, thetransition time determination unit 34 c is not always a necessarycomponent.

Embodiments and the modification examples thereof have been describedabove with reference to the specific examples. Embodiments describedabove are merely given as examples, and are not limited. In addition,the components of embodiments may be combined as appropriate.

In embodiments and the modification examples thereof, the case where thecarrier gas, the source gas, the dopant gas, and the assist gas areindependently supplied into the reactor 10 (FIG. 1 ), the case where twoor more of the carrier gas, the source gas, the dopant gas, and theassist gas are mixed (FIG. 9 ), the case where all of the carrier gas,the source gas, the dopant gas, and the assist gas are mixed to besupplied into the reactor 10 (FIG. 8 ), and the like have been describedas examples. However, other methods may be used.

In embodiments and the modification examples thereof, the case where thesilicon carbide layer contains an n-type impurity as a dopant has beendescribed as an example. However, the silicon carbide layer may containa p-type impurity as a dopant.

In embodiments and the modification examples thereof, the description ofparts that are not directly required for the description of theinventions, such as the apparatus configuration or the manufacturingmethod, is omitted. However, the required apparatus configuration,manufacturing method, and the like can be appropriately selected andused. In addition, all vapor phase growth methods and vapor phase growthapparatuses that include the elements of the inventions and that can beappropriately redesigned by those skilled in the art are included in thescope of the inventions. The scope of the inventions is defined by thescope of claims and the scope of their equivalents.

What is claimed is:
 1. A vapor phase growth method, comprising: forminga first silicon carbide layer having a first doping concentration on asilicon carbide substrate at a first growth rate by supplying a firstprocess gas containing a carrier gas into a reactor under a first gascondition; forming a second silicon carbide layer having a second dopingconcentration at a second growth rate higher than the first growth rateby supplying a second process gas containing a carrier gas into thereactor under a second gas condition after the forming the first siliconcarbide layer; and forming a third silicon carbide layer having a thirddoping concentration lower than the first doping concentration and thesecond doping concentration at a third growth rate higher than thesecond growth rate by supplying a third process gas containing a carriergas into the reactor under a third gas condition after the forming thesecond silicon carbide layer.
 2. The vapor phase growth method accordingto claim 1, wherein a thickness of the second silicon carbide layer islarger than a thickness of the first silicon carbide layer, and athickness of the third silicon carbide layer is larger than thethickness of the second silicon carbide layer.
 3. The vapor phase growthmethod according to claim 1 further comprising: forming a firsttransition layer during a first transition time for switching from thefirst gas condition to the second gas condition after the forming thefirst silicon carbide layer; and forming a second transition layerduring a second transition time for switching from the second gascondition to the third gas condition after the forming the secondsilicon carbide layer, wherein an average residence time of the carriergas in the reactor when forming the first transition layer is shorterthan the first transition time, and wherein an average residence time ofthe carrier gas in the reactor when forming the second transition layeris shorter than the second transition time.
 4. The vapor phase growthmethod according to claim 1, wherein a carbon/silicon atom ratio in thesecond process gas is smaller than a carbon/silicon atom ratio in thefirst process gas, and a carbon/silicon atom ratio in the third processgas is larger than the carbon/silicon atom ratio in the second processgas.
 5. The vapor phase growth method according to claim 1, wherein asilicon/hydrogen atom ratio (Si/H) in the second process gas is largerthan a silicon/hydrogen atom ratio (Si/H) in the first process gas.
 6. Avapor phase growth apparatus, comprising: a reactor; a carrier gassupply pipe supplying a carrier gas to the reactor; a first source gassupply pipe supplying a first source gas containing silicon (Si) to thereactor; a second source gas supply pipe supplying a second source gascontaining carbon (C) to the reactor; a first mass flow controllerprovided in the carrier gas supply pipe to control a flow rate of thecarrier gas supplied to the reactor; a second mass flow controllerprovided in the first source gas supply pipe to control a flow rate ofthe first source gas supplied to the reactor; a third mass flowcontroller provided in the second source gas supply pipe to control aflow rate of the second source gas supplied to the reactor; a pressureadjusting valve adjusting a pressure in the reactor; and a controlcircuit controlling the first mass flow controller, the second mass flowcontroller, the third mass flow controller, and the pressure adjustingvalve so that an average residence time of the carrier gas in thereactor is shorter than a transition time when switching the flow rateof at least one of the first source gas and the second source gassupplied to the reactor at the transition time.
 7. The vapor phasegrowth apparatus according to claim 6, wherein the control circuitincludes an average residence time calculation circuit for calculatingthe average residence time based on a pressure of the reactor, a volumeof the reactor, and the flow rate of the carrier gas.
 8. The vapor phasegrowth apparatus according to claim 7, wherein the average residencetime calculation circuit calculates the average residence time bydividing a value obtained by multiplying the pressure of the reactor bythe volume of the reactor by the flow rate of the carrier gas.